The present invention generally relates to a flexible cable for coupling a flat panel display with a data processing unit. Specifically, the invention relates to a flexible cable that transmits low voltage differential signals and promotes reduced levels of emissions of electromagnetic radiation.
Portable computer displays receive power and electrical signals from the base portion of the portable computer via a flexible cable or circuit board interconnect. The cable or interconnect should be stable and robust enough to withstand many wear and fatigue cycles due to repeated opening and closing cycles without degrading in performance. Where the data rate of transmission with the display is high, such as with low voltage differential signaling (LVDS) used in connection with high-speed video signals, shielding must be supplied around the flexible circuit to protect against electromagnetic interference (EMI). EMI shielding is usually provided by solid ground planes that encase the signaling lines of the cable.
In order to reduce transmission errors induced by noise during data transmission, the cable is usually powered at voltage levels below 3 volts and exhibits impedance levels of over 100 ohms. The ground planes that shield the cable from emitting EMI to the surrounding computer components also control the impedance of the cable. However, use of the solid ground planes reduces cable flexibility and leads to eventual failure of the cable through fatigue caused by the repetitive opening and closing of the portable computer display.
Although it is preferable in low voltage differential signaling to increase the impedance in the cable, the solid ground planes tend to reduce the impedance of the cable. Cable impedance is reduced because of the large capacitance that is generated in the small vertical spacing between the signal lines and the solid ground plane. One approach to increasing the impedance in the cable is to increase the spacing between the signal lines and the solid ground plane, thereby reducing the capacitance. However, the increased spacing increases the overall thickness of the cable and reduces the cable""s flexibility. Although operative, such cable construction is impractical for use in certain applications, such as in various portable computers because of the cable size and its predisposition for failure through fatigue.
A cable arrangement and a data processing system that addresses the aforementioned problems, as well as other related problems, are therefore desirable.
Various embodiments of the present invention are directed to addressing the above and other needs involving a flexible interconnect arrangement for electrically connecting a display unit to a data processing unit that facilitates the transmission of low voltage differential signals and inhibits electromagnetic interference (EMI) along the length of the interconnect arrangement In addition, the flexible interconnect arrangement includes a grounding plane arrangement that results in a thin, highly flexible structure that is resist to early breakdown as a result of fatigue.
According to one embodiment of the invention, a flexible interconnect arrangement transmits low voltage differential signals along a length of the flexible interconnect arrangement and inhibits electromagnetic radiation generated by the signals that travel along the length of the flexible interconnect arrangement. The flexible interconnect arrangement includes a conductive layer extending along a plane in the direction of the length of the flexible interconnect arrangement and an upper insulative layer and a lower insulative layer respectively above and below the conductive layer. The interconnect arrangement further includes an upper shield layer and lower shield layer above and below the upper insulative layer and the lower insulative layer. The flexible interconnect arrangement layers form at least first and second opposing sides orthogonal to the plane of the conductive layer. The interconnect arrangement also includes at least two conductive shield members that face the first and second opposing sides of the flexible interconnect arrangement layers and electrically connect the upper shield layer and the lower shield layer on each opposing side. The conductive shield members and the shield layers inhibit EMI about the length of the flexible interconnect arrangement.